Boiling water reactor nuclear fuel in suitable compacted form is usually enclosed in corrosion-resistant, non-reactive, heat-conductive containers or cladding which in assembly may take the form of rods, tubes or plates. A plurality of fuel elements of this kind are assembled in a fixed spaced relation in a coolant flow channel, and a number of these assemblies are combined to form a reactor core capable of a self-sustained fission reaction. The core is contained in a reactor vessel through which water as a coolant is run continuously.
A prime necessity in the operation of a nuclear reactor is the containment of radioactive fission products. The cladding serves this purpose, preventing release of those products into the coolant and, in addition, preventing contact and chemical reaction between the nuclear fuel and the coolant. Common cladding materials include zirconium and its alloys, particularly Zircaloy-2 and Zircaloy-4.
During operation of a nuclear powered reactor, a fissionable atom of U-233, U-235, Pu-239 or Pu-241 undergoes a nuclear disintegration producing an average of two fission products of lower atomic weight and great kinetic energy. Some of such fission products, including iodine and bromine, have been found or considered to have corrosive effects on the cladding. Thus, cladding failure resulting from such corrosion has been observed during operation of nuclear reactors over long periods of time.
As disclosed and claimed in U.S. Pat. No. 3,826,754, assigned to the assignee hereof, certain additives can be incorporated in nuclear fuels to prevent corrosive attack on cladding by fission products. This result is achieved without offsetting disadvantage by chemical combination or association of the additives with deleterious fission products whereby those fission products are prevented from migrating in the nuclear fuel to reach the cladding.